1. Field of the Invention
The present invention relates to a solid state imaging device and a method for manufacturing the same. In particular, it relates to a solid state imaging device including a plurality of light receiving elements arranged in a matrix and a method for manufacturing the same.
2. Description of Related Art
In recent years, there has been an increasing demand for solid state imaging devices as imaging devices for digital still cameras and video cameras. Further, in the field of mobile devices such as cellular phones, camera-equipped ones are highly demanded. From this aspect, the demand for the solid state imaging devices has been expanding. Aiming at higher quality images, the number of pixels tends to increase year after year. As the increase in pixel count involves the need of decreasing pixel area, further improvement in light gathering power has been demanded.
Now, referring to FIGS. 7 to 9, explanation of a conventional solid state imaging device is provided. FIG. 7 is a plan view schematically illustrating the structure of a conventional CCD (charge coupled device) solid state imaging device. As shown in FIG. 7, the CCD solid state imaging device includes a semiconductor substrate 201 on which a plurality of light receiving elements 202 are provided in a two-dimensional arrangement. Further, vertical transfer portions (vertical CCDs) 203 are provided between the vertical columns of the light receiving elements 202 and a horizontal transfer portion (horizontal CCD) 204 is provided adjacent to the endmost line of the light receiving elements 202. The light receiving elements 202 are photodiodes and accumulate charges corresponding to the intensity of light received. A single light receiving element 202 and part of the vertical CCD 203 adjacent thereto constitute a single pixel 206.
As indicated by an arrow in FIG. 7, the charges accumulated in the light receiving elements 202 are read out and transferred in the vertical direction by the vertical CCDs 203. The charges transferred by the vertical CCDs 203 are then transferred in the horizontal direction by the horizontal CCD 204, amplified by an amplifier 205 and then externally output.
In order to improve the light gathering power of the light receiving elements 202, a combination of a microlens and an optical waveguide is provided on each of the light receiving elements 202. Hereinafter, explanation of the CCD solid state imaging device is provided with reference to FIGS. 8 and 9. FIG. 8 is a plan view illustrating an enlargement of part of the conventional solid state imaging device provided with the microlenses and the optical waveguides and FIG. 9 is a sectional view taken along the line IX-IX shown in FIG. 8.
In the CCD solid state imaging device, as shown in FIG. 8, vertical CCDs 103, openings 107, optical waveguides 108 and on-chip microlenses 109 are provided on a semiconductor substrate 111. The vertical CCDs 103, openings 107, optical waveguides 108 and on-chip microlenses 109 constitute unit cells 106. As shown in FIG. 9, charge transfer portions 114 and photodiodes 115 are also formed in the semiconductor substrate 111. The surface of the semiconductor substrate 111 is covered with a gate insulating film 112 and gate electrodes 113 are formed on the gate insulating film 112. An interlayer insulating film 116 is formed on the gate electrodes 113 and a light shield film 117 is formed thereon to cover the gate insulating film 112 and the interlayer insulating film 116. The openings 107 are formed in the light shield film 117.
An insulating film 118 is formed on the light shield film 117 and in the openings 107. The optical waveguides 108 are formed in the openings 107. On the insulating film 118 and the optical waveguides 108, a planarization film 121 is formed with a passivation film 126 interposed therebetween. A color filter 122 is formed on the planarization film 121 and the on-chip microlenses 109 are formed on the color filter 122 to be positioned above the photodiodes 115.
As described above, the CCD solid state imaging device shown in FIG. 9 includes the on-chip microlenses 109 formed at the topmost surface and the optical waveguides 108 formed in the insulating film 118. As the optical waveguides 108 and the microlenses 109 are provided at positions corresponding to the photodiodes 115, the light gathering power of the photodiodes 115 improves to a further extent (see Japanese Unexamined Patent Publication No. H11-121725 (page 7, FIG. 1)).